Field of the Invention
The invention relates to a system for triggering a restraining device in a motor vehicle with an evaluation device in which an impact signal supplied by a sensor device for detecting accidents is evaluated. An ignition device is disposed spatially separated from the evaluation device in the motor vehicle and electrically connected to the evaluation device via a line, wherein an alternating signal containing a message is transmitted from the evaluation device to the ignition device and is evaluated in a logic circuit of the ignition device. The restraining device is triggered by an ignition element, which is electrically connected to the ignition device. An ignition capacitor in the ignition device makes available the energy for firing the ignition element.
Such a configuration is described in European patent EP 0 471 871 B1. The prior art system has an evaluation device in which a signal supplied by a sensor device for detecting accidents is evaluated. A plurality of ignition devices distributed spatially about the vehicle are electrically connected to the evaluation device via a line or a bus. The evaluation device supplies to an ignition device alternating signals which contain messages and which are evaluated in a logic circuit of the ignition device. Each ignition device is electrically connected to an ignition element of a restraining device. In addition, it is proposed to supply the power necessary for operating the ignition devices to the ignition devices via the line.
A widely conventional data transmission technique makes use of the (amplitude, phase or frequency) modulation of a high-frequency carrier signal with a code signal. However, the application of such a data transmission system in the prior art configuration for triggering a restraining device is disadvantageous since the modulated carrier signal must supply a sufficient quantity of energy to operate the ignition device. Owing to the dependence of the transmitted energy on the data it is not possible to ensure a constant energy supply.
In addition, with such a data transmission method, the maximum transmittable data rate is limited by the frequency of the carrier signal. Due to the fact, however, that trigger instructions for triggering the restraining device are also transmitted, an extremely short data transmission time, and thus a very high data rate, are required for correctly timed triggering of the restraining device (airbag, belt retractor, and the like). However, a high data rate requires a high carrier frequency. On the other hand, the higher the carrier frequency selected, the stronger the radiated interference from the line. The disadvantageous effect of strong radiated interference on electrical circuits is well known. This effect is amplified by the fact that a considerable amount of energy for operating the ignition device is transmitted together with the modulated carrier signal.